The invention will be now specifically described in reference to the use of modified zinc oxide as the UV attenuating oxide, however the invention also extends to other inorganic oxides including titanium dioxide and Iron oxide.
Zinc oxide scatters some wavelengths of light and absorbs other selected wavelengths of light. It exhibits very strong absorption at wavelengths just short of the visible spectrum. It Is a strong UVB absorber at the 210-320 nm wavelength and also at the UVA 320-840 nm wavelength. At wavelengths longer than 370 nm. protection is provided by scattering and at wavelengths shorter than 370 nm, protection is achieved predominantly by absorption. Zinc oxide exhibits a strong semi-conductor absorption in the ultraviolet region. The optimum size of a zinc oxide particle for attenuation of the ultraviolet radiation is less than 0.06 .mu.m for the wavelength range 300-400 nm. Absorption is the dominant mechanism for attenuation of ultraviolet radiation in the wavelength range of 300-400 nm.
Zinc oxide has been used as a white reflective pigment (particles size 1.0-20 .mu.m) and as a UV absorber (particles 0.3-5 .mu.m) for many years. The earliest form of presentation of zinc oxide as a sunscreen agent is a pigmented cream which is used to protect various parts of the exposed body, particularly the nose. Although this zinc oxide cream has its place, the conspicuous nature of the cream when on the skin reduces Its cosmetic appeal significantly. Attempts at turning a disadvantage to advantage has led to recent variations of the zinc oxide cream in which a coloured pigment is used to produce a brightly colour cream such as blue, red, yellow or green cream. Such modifications, although popular particularly with the young, have limited cosmetic appeal to the more general population.
It is highly desirable that the UV attenuating substance be invisible on the skin with any colour variation in the sunscreen formulation being adjusted to achieve a desired cosmetic effect.
To overcome this visual problem and also improve the performance as a sunscreen, zinc oxide with a particle size of 0.1-1.0 .mu.m has been used in order to increase UV absorption and decrease reflectance of light. Moreover zinc oxide with a particle size less than 0.1 .mu.m becomes invisible when rubbed onto the skin. However prior art attempts to use microfine zinc oxide has led to difficulties in formulation. The fine zinc oxide powder is difficult to keep in suspension. The particles irreversibly bond during manufacture, and when formulated into a dispersion, tend to aggregate, agglomerate and then settle out. Prior art sunscreen creams incorporating zinc oxide in this particle size range appear white on the skin, develop a settled out layer which is difficult to disperse. Moreover the prior art formulations tend to develop a clear top layer.
Prior art zinc oxide formulations are also limited in terms of the amount of zinc oxide that could be mixed into the formulation (up to about 10% by weight) which limits the blockout effect of the sunscreen. To disperse the zinc oxide in the formulation, various wetting agents, in combination with anti-settling agents have been used, this effectively decreasing the amount of UV active that can be included in the formulation, adding to the cost of manufacture and increasing the risk of an adverse skin reaction.
When zinc oxide is freshly made into a fine powder by burning fumed zinc metal vapour, oxygen will bond onto the fresh zinc oxide surfaces. This oxygen bonding then prevents any further bonding with hydrogen or oxygen ions when the zinc oxide comes in contact with an ester or alcohol and means that this zinc oxide is prone to agglomeration, aggregation, settling and hence whitening.
Furthermore, freshly made zinc oxide can absorb carbon dioxide which leads to the formation of carbonates on its surface. This absorption of carbon dioxide thus inhibits the UV absorbing properties of the zinc oxide.